If the start time for your hearing has changed but is on the same day as your approved reservation, you don't need to do anything. Just dial in at the new time and your reservation will work without any modifications.
If the date of your hearing has changed, you need to make a new reservation. See here for how to make a new reservation.
You can cancel your old reservation by clicking the trash can next to your prior reservation.
Please note that if you do not dial in for your reservation, you will NOT be charged.